A Micro-Electro-Mechanical Systems (MEMS) gyroscope sensor consists of one or more movable proof masses connected to each other and to one or more substrates by flexible suspensions. Typically, the proof masses and suspensions are fabricated by etching heavily doped silicon, and the silicon is bonded to one or more upper and/or lower glass or silicon substrates.
The proof masses are driven electrostatically at the resonant frequency of the “motor” mode. When the sensor experiences rotation, the velocity of the motor mode motion causes the proof masses to experience Coriolis forces perpendicular to the motor velocity and the rotation axis. The proof mass motion produced by the Coriolis force is sensed capacitively, by sense electrodes, to produce an electrical output signal.
In various MEMS gyroscopes, the motor mode consists of the two proof masses moving with equal and opposite velocities parallel to the substrate and along a line connecting the centers of the two proof masses. The motor mode resonant frequency can be in the range of 10 to 20 kHz. MEMS gyroscope sensors may be designed to sense rotation either parallel or perpendicular to the substrate. The Coriolis force drives the “sense” resonant mode of the silicon mechanism, which consists of the two proof masses moving in opposing directions parallel or perpendicular to the substrate, depending on whether the rotation axis is perpendicular or parallel to the substrate. A sensor designed to sense rotation around an axis parallel to the substrate is referred to as an in-plane gyroscope (IPG), and a sensor designed to sense rotation around an axis perpendicular to the substrate is referred to as a z-axis gyroscope or out-of-plane gyroscope (OPG).
In a MEMS gyroscope, the resonant frequency of the sense mode is typically 5% to 10% below the resonant frequency of the motor mode, so the Coriolis force drives the sense mode off-resonance. Other MEMS gyroscopes may operate with the sense resonant frequency as close as possible to the motor resonant frequency, in order to maximize scale factor. However, the bandwidth of such sensors is very limited, and they can have stability issues.
Other MEMS gyroscopes may be quite different from the two-proof mass configuration described above. However, they all have a motor mode driven at its resonant frequency, they all experience a Coriolis force during rotation, and the Coriolis force drives a sense mode whose motion is detected capacitively.
The sensor output signal in MEMS gyroscopes is an AC signal at the motor resonant frequency. Typical MEMS gyroscopes have a large output error signal, designated “quadrature”, which is 90 degrees out-of-phase with the output signal produced by the Coriolis force. Phase-sensitive detection allows the Coriolis rate signal to be detected in the presence of a much larger quadrature signal. However, phase shifts in the electronics and in the sensor can cause the quadrature-phase signal to produce errors in the Coriolis-phase signal.
Parametric amplification in a MEMS gyroscope using a pump signal consisting of AC voltages applied to the sense electrodes has been described previously, in U.S. Pat. No. 6,715,353 (Burgess R. Johnson, issued Apr. 6, 2004), which is incorporated by reference in its entirety herein. The AC pump voltages can increase the mechanical gain of the sensor (transfer function from input force to sensor mechanism displacement), as well as the electrical gain of the sensor (transfer function from sensor mechanical displacement to output electrical signal). Also, the mechanical and electrical gains of the sensor become phase-dependent, so that the Coriolis rate signal can be amplified while the unwanted quadrature-phase signal is attenuated.
Oropeza-Ramos, et. al., have described the use of parametric resonance to modify the characteristics of the driven mode of a MEMS gyroscope (“Parametric Resonance Amplification in a MEM Gyroscope,” L. A. Oropeza-Ramos and K. L. Turner, Proceedings of the 2005 IEEE Sensors Conference, pages 660-663, Oct. 31-Nov. 3, 2005, Irvine, Calif.), incorporated by reference in its entirety herein.
A difficulty with using AC pump voltages to provide parametric amplification, as described in U.S. Pat. No. 6,715,353, is that the phase of the pump voltages must be precisely synchronized with the phase of the driven motion of the sensor mechanism. If this is not done, the parametric amplification produces a phase shift of the sensor output signal relative to the phase of the input force on the sensor. As a result, the relatively large quadrature-phase force on the sensor mechanism can produce sensor output at the Coriolis rate phase, resulting in a large zero-rate bias error.